For an optical system, if it is free from optical aberrations, then its spatial resolution is limited by optical diffraction. Optical diffraction is well known phenomenon. When light passes through an aperture, the light spreads out so that a point light sources becomes a pattern of light across the director known as diffraction pattern. Diffraction pattern depends on size and shape of the optical aperture. For example, a circular aperture generates a diffraction pattern known as Airy disc, which has a bright central spot, surrounded by fainter bright and dark rings. These rings gradually fade as the diameter increases. When the center bright spot of the Airy disc becomes large relative to the pixel spacing of detector, the image looks burr. If two Airy disks become closer than half their width, the two spots are no longer resolvable (Rayleigh criterion). An optical system with the ability to produce images with angular separations as small as the instrument's theoretical limit is called diffraction limited.
A basic limitation of any optical systems is the aperture size. High resolution optical systems require very larger apertures. In many situations the large aperture optical systems are very costly, especially for space based optical systems.
Deconvolution is specifically used to refer to the process of reversing the optical distortion that takes place in an optical imaging instrument, and creating sharper images. Deconvolution is usually done by software algorithm. Generally speaking, if a point spread function (PSF) can be found, we can compute its inverse function, and restore the original, undistorted image. But in practice, the inverse of the PSFs may become infinite in some areas, and the noise level may become unacceptable. The Optical Transfer Function (OTF) describes the spatial (angular) variation as a function of spatial (angular) frequency. The OTF is the Fourier transform of the PSF.
It is desirable for obtain high resolution images with small aperture optical systems. There are a few imaging techniques can claim to increase image resolution beyond diffraction limit. Super-resolution (SR) is one technique that in some way can enhance the resolution of an imaging system. There are some SR techniques that process images in the frequency domain, by assuming that the object on the image is an analytic function, and that we can exactly know the function values in some interval. SR method needs multi-frame images and is limited by the noise that is present in digital imaging systems.